1. Field of the Invention
The present invention relates generally to saboted projectiles and, more particularly, to a method of improving the in-bore performance of kinetic energy projectiles. By better controlling the interior ballistic processes of the projectile, adverse dynamic perturbations to the projectile during launch are minimized. Most particularly, the in-bore performance of kinetic energy projectiles is improved through modification of the projectile's bourrelet, thus providing improved shot accuracy for the projectiles.
2. Description of the Related Art
Kinetic energy ammunition is comprised of two main parts: the projectile portion that flies down the gun tube; and the portion that remains in the gun or is combusted during firing (cartridge case, case base, base adapter and propellant). The projectile further comprises several parts including: the sabot; the obturator; the seal; and the sub-projectile penetrating rod. As such, the sabot is part of the launch package that carries the sub-caliber projectile, such as a long-rod penetrator, during bore travel in the gun barrel when fired.
Sabots serve several important functions during the short time spent inside the gun bore (roughly 6 milliseconds of in-bore time for the M256 120-mm tank cannon), after which, at the end of the gun tube, the sabot must be gently discarded in order not to interfere with the flight of the sub-projectile. First, the sabot, along with the gas seal and obturator, must seal the gun bore to contain the high-pressure propellant gases between the rod and the gun tube. Second, the sabot must support the sub-projectile during high axial acceleration and load during launch. In fact, the axial acceleration and loads during launch are high enough to fail the penetrator rod in either compression or tension. Third, the sabot provides the suspension which controls the ride of the projectile down the gun tube. Finally, the sabot must also gently discard from the projectile soon after muzzle exit without disturbing the sub-projectile flight. The only part of the projectile that reaches the target is the sub-projectile rod. This means that any other part of the projectile (the sabot, obturator, and seals) is parasitic in nature. Since the effectiveness of the rod is directly related to the velocity on target, it is paramount that the sabots, seal, and obturator are as light as possible so they do not reduce the kinetic energy available to the rod.
The sabot typically includes a forward bore riding bourrelet or support and an aft bore riding bourrelet or support. Although all sabots do not have both a forward and an aft bourrelet, all sabots must have at least one bourrelet in order to function properly. The aft bourrelet is a solid cylindrical structure which, along with the obturator, forms a seal for propellant gases. The propelling gases push against the aft face of the aft bourrelet to push the projectile through the gun tube. The forward bourrelet is spaced apart from the aft bourrelet and includes a scoop to catch onrushing air upon projectile exit from the gun tube. The scoop-shaped forward bourrelet thus permits the onrushing air to separate the segments or petals of the sabot from the fin stabilized long rod penetrator upon flight.
During projectile ride down the gun tube, the front and aft bourrelets contact the inner surface of the gun tube. These bourrelet surfaces act as the suspension for the projectile during launch. To improve and ensure shot accuracy, the dispersion of the projectile with respect to the fire precision must be decreased. Dispersion is the area covered by a group of shots at the target.
When a projectile is fired it usually does not go exactly where the gun is aimed for a variety of reasons. The vector from the line of fire to the impact on the target is defined as the projectile "jump." Projectile jump is comprised of a number of components that correlate to different aspects of launch and flight. The present invention is directed towards tuning the in-bore performance of projectiles so that shot accuracy is improved by minimizing the projectile's transverse linear velocity and transverse angular rates relative to the gun bore at shot exit from the gun tube. The present invention fulfills this need for both new projectiles being developed and for ammunition already in production through modification of the forward and/or aft bourrelets. It has been shown that changing the stiffness of the bourrelets can minimize transverse velocity and angular spin rates thus minimizing shot dispersion for saboted projectiles. The ability to control the interior ballistic processes to minimize adverse dynamic perturbations to the projectile during the launch represents a major step toward "designing in" accuracy for projectiles.